<!DOCTYPE HTML>
<style type="text/css">
html, body { margin: 0; padding: 0; border: none; }
</style>

<div style="margin: 100px; height: 400px; width: 400px; display: -moz-box; -moz-box-orient: horizontal">

  <div style="width: 200px; box-shadow: 0 0 14px blue">A</div>
  <div style="width: 100px"></div>
  <div style="-moz-box-flex: 1; box-shadow: 0 0 14px blue">B</div>

</div>
